You never have to save your responses. When you are done answering a question, navigate to the previous or next question, or return to the Questionnaire (Section listing), and the system will save your work automatically. In each case you will see a small pop-up telling you that the response has been saved.
If you do not wish to navigate forwards or backwards, you can tap the checkmark icon at the top right area of the reponse screen instead to save your response. This action will return you to the audit questionnaire screen.